Global production of farmed fish and crustacea has more than doubled in the last 15 years and its expansion places an increasing demand on global supplies of wild fish harvested to provide protein and oil as ingredients for aquafeeds (Naylor et al., 2000). The supply of seafood from global capture fisheries sources is around 100 million tones per annum (FAO, 2001). This amount has not increased since the mid-1980's and will not increase in the future as most fisheries are at or above sustainable levels of production, and are further subjected to sharp, periodic declines, due to climatic factors such as El Niño (FAO, 2001; Barlow 2000). Fish oil stocks are also under increasing demand not only from aquaculture, but from the agriculture and nutraceutical/biomedical industries.
Replacement oils for the aquaculture industry have been sourced from a variety of commercial terrestrial plant sources including sunflower (Bransden et al, 2003; Bell et al., 1993), canola/rapeseed (Bell et al, 2003; Polvi and Ackman, 1992), olive, palm (Fonseca-Madrigal et al, 2005; Bell et al, 2002) and linseed (Bell et al., 1993; Bell et al., 2004). The inclusion of vegetable oil to replace part or all of the fish oil in fish diets resulted in the same growth rates and feed conversion ratios (Bransden et al., 2003; Polvi and Ackman, 1992; Torstensen et al., 2004; Fonseca-Madrigal et al., 2005; Bell et al., 2002; Bell et al., 2004). However, since these plant oils had essentially no ω3 long-chain (≧C20) polyunsaturated fatty acids (ω3 LC-PUFA) and had high levels of monounsaturated fatty acids (MUFA), ω6 PUFA and low ω3/ω6 ratios, fish fed such diets displayed reduced levels of ω3 LC-PUFA. This is thought to be associated with reduced health benefits to the consumer compared to fish fed a diet high in fish oil containing greater levels of ω3 LC-PUFA (Seierstad et al., 2005). Therefore, raising fish or crustacea on diets high in vegetable oil has the potential to dilute the important cardiovascular and other benefits which are associated with eating fish.
Pathways of LC-PUFA Synthesis
Biosynthesis of LC-PUFA from linoleic and α-linolenic fatty acids in organisms such as microalgae, mosses and fungi may occur by a series of alternating oxygen-dependent desaturations and elongation reactions as shown schematically in FIG. 1. In one pathway (FIG. 1, II), the desaturation reactions are catalysed by Δ6, Δ5, and Δ4 desaturases, each of which adds an additional double bond into the fatty acid carbon chain, while each of a Δ6 and a Δ5 elongase reaction adds a two-carbon unit to lengthen the chain. The conversion of ALA to DHA in these organisms therefore requires three desaturations and two elongations. Genes encoding the enzymes required for the production of DHA in this aerobic pathway have been cloned from various microorganisms and lower plants including microalgae, mosses, fungi.
Alternative routes have been shown to exist for two sections of the ALA to DHA pathway in some groups of organisms. The conversion of ALA to ETA may be carried out by a combination of a Δ9 elongase and a Δ8 desaturase (the so-called Δ8 desaturation route, see FIG. 1, IV) in certain protists and thraustochytrids, as evidenced by the isolated of genes encoding such enzymes (Wallis and Browse, 1999; Qi et al., 2002). In mammals, the so-called “Sprecher” pathway converts DPA to DHA by three reactions, independent of a Δ4 desaturase (Sprecher et al., 1995).
Besides these desaturase/elongase systems, EPA and DHA can also be synthesized through an anaerobic pathway in a number of organisms such as Shewanella, Mortiella and Schizochytrium (Abbadi et al., 2001). The operons encoding these polyketide synthase (PKS) enzyme complexes have been cloned from some bacteria (Morita et al., 2000; Metz et al., 2001; Tanaka et al., 1999; Yazawa, 1996; Yu et al., 2000; WO 00/42195). The EPA PKS operon isolated from Shewanella spp has been expressed in Synechococcus allowing it to synthesize EPA (Takeyama et al., 1997). The genes encoding these enzymes are arranged in relatively large operons, and their expression in transgenic plants has not been reported. Therefore it remains to be seen if the anaerobic PKS-like system is a possible alternative to the more classic aerobic desaturase/elongase for the transgenic synthesis of LC-PUFA.
The biosynthetic pathways for PUFA are well known (Sargent et al., 2002). Vertebrates lack ω12 and ω15 (ω3) lipid desaturases and cannot produce linoleic acid (18:2 ω6, LA) and α-linolenic acid (18:3ω3, ALA) from oleic acid (18:1ω9, OA) (see FIG. 1). The conversion from ALA to eicosapentaenoic acid (20:5ω3, EPA) and docosahexaenoic acid (22:6ω3, DHA) is inefficient in marine fish, which have high levels of LC-PUFA in their natural diet, but is greater in freshwater fish, which have high levels of LA and ALA and limited DHA in their natural diet. High levels of ω3 LC-PUFA, which are found in salmon, cannot be biosynthesised from ALA and LA and therefore must be provided to the fish in their diet.
Desaturases
The desaturase enzymes that have been shown to participate in LC-PUFA biosynthesis all belong to the group of so-called “front-end” desaturases which are characterised by the presence of a cytochrome b5 domain at the N-terminus of each protein. The cyt b5 domain presumably acts as a receptor of electrons required for desaturation (Sperling and Heinz, 2001). The enzyme Δ6 desaturase catalyses the desaturation of linoleic acid (LA) to form gamma-linoleic acid (GLA, 18:3 ω6) and linolenic acid (ALA) to form stearidonic acid (SDA, 18:4ω3) (FIG. 1). Genes encoding this enzyme have been isolated from a number of organisms, including plants, mammals, nematodes, fungi and marine microalgae. The C18 fatty acid substrate for Δ6 desaturases from plants, fungi and microalgae has desaturation in at least the Δ9 and Δ12 positions and is generally covalently linked to a phosphatidylcholine headgroup (acyl-PC).
The enzyme Δ5 desaturase catalyses the desaturation of C20 LC-PUFA leading to arachidonic acid (ARA, 20:4 ω6) and EPA (20:5ω3). Genes encoding this enzyme have been isolated from a number of organisms, including algae (Thraustochytrium sp. Qiu et al., 2001), fungi (M. alpine, Pythium irregulare, Michaelson et al., 1998; Hong et al., 2002), Caenorhabditis elegans and mammals. A gene encoding a bifunctional Δ5-/Δ6-desaturase has also been identified from zebrafish (Hasting et al., 2001). The gene encoding this enzyme might represent an ancestral form of the “front-end desaturase” which later duplicated and evolved distinct functions.
The last desaturation step to produce DHA is catalysed by a Δ4 desaturase and a gene encoding this enzyme has been isolated from the freshwater protist species Euglena gracilis and the marine species Thraustochytrium sp. (Qiu et al., 2001; Meyer et al., 2003).
Elongases
Several genes encoding PUFA-elongation enzymes have also been isolated (Sayanova and Napier, 2004). The members of this gene family were unrelated to the elongase genes present in higher plants, such as FAE1 of Arabidopsis, that are involved in the extension of saturated and monounsaturated fatty acids. An example of the latter is erucic acid (22:1) in Brassicas. In some protist species, LC-PUFA are synthesized by elongation of linoleic or α-linolenic acid with a C2 unit, before desaturation with Δ8 desaturase (FIG. 1 part IV; “Δ8-desaturation” pathway). Δ6 desaturase and Δ6 elongase activities were not detected in these species. Instead, a Δ9-elongase activity would be expected in such organisms, and in support of this, a C18 Δ9-elongase gene has recently been isolated from Isochrysis galbana (Qi et al., 2002).
Transgenic Plants
Transgenic oilseed crops that are engineered to produce major LC-PUFA by the insertion of various genes encoding desaturases and/or elongases have been suggested as a sustainable source of nutritionally important fatty acids. However, the requirement for coordinate expression and activity of five new enzymes encoded by genes from possibly diverse sources has made this goal difficult to achieve and only low yields have generally been obtained (reviewed by Sayanova and Napier, 2004; Drexler et al., 2003; Abbadi et al., 2001).
A gene encoding a Δ6-fatty acid desaturase isolated from borage (Borago officinalis) was expressed in transgenic tobacco and Arabidopsis, resulting in the production of GLA (18:3ω6) and SDA (18:4 ω3), the direct precursors for LC-PUFA, in the transgenic plants (Sayanova et al., 1997 and 1999). However, this provides only a single, first step.
Feedstuffs for Aquaculture
Research in feedstuffs for aquaculture have largely focused on enriching salmon diets by increasing the dietary supply of ALA (Bell et al., 1993) and EPA/DHA (Harel et al., 2002; Carter et al., 2003).
There is a need for further diets for aquaculture which, upon consumption, enhance the production of omega-3 long chain polyunsaturated fatty acids in aquatic animals.